The use of plastic housings for electronic equipment and components is widely accepted in the automotive and electronic equipment fields today. However, the presently available plastic materials suffer from the disadvantage of being transparent or permeable to electromagnetic interference commonly known as, and referred to, as EMI. This drawback in available plastic materials is a matter of considerable concern in view of the susceptibility of electronic equipment to the adverse effects of EMI emission by the growing number of consumer products which produce such EMI signals and to the increasing regulatory controls exercised over such electromagnetic pollution.
Currently, the major approach to solving plastic material shielding problems is through the application of metallic surface coatings to the molded plastic. Among such approaches are the use of vacuum deposition, metal foil linings, metal-filled spray coatings, zinc flame-spray and electric arc discharge. Each of these procedures is accompanied by one or more drawbacks with respect to cost, adhesion, scratch resistance, environmental resistance, the length of time required for application and the difficulties in adequately protecting many of the diverse geometrical forms in which the molded plastic must be provided.
More recently, attempts have been made to resolve the problem of EMI by formulation of composite plastic materials based upon the use of various fillers in thermoplastic matrices. Among the conductive fillers which have been employed for this purpose are carbon black, carbon fibers, silver coated glass beads and metallized glass fibers. However, these materials are subject to the disadvantages of being brittle to the extent that they break up into shorter lengths in processing. The shorter length fibers and particles require higher loadings or filler concentrations leading to embrittlement of the plastic matrix and higher costs which render them commercially unacceptable. Hence, none of the composite plastic products developed heretofore have proven completely satisfactory.